What are the best practices for spectral gamma ray logging in horizontal wells and multilateral completions?

1 Principles of SGR logging

SGR logging is based on the detection of gamma rays emitted by the decay of naturally occurring isotopes of potassium (K), thorium (Th), and uranium (U) in the rock. Different rock types and minerals have different concentrations of these elements, and thus different gamma ray signatures. By measuring the energy spectrum of the gamma rays, SGR logging can distinguish between K, Th, and U sources and quantify their relative contributions. SGR logging can also identify organic matter by its low gamma ray response, as organic matter typically has low concentrations of K, Th, and U.

2 Benefits of SGR logging in horizontal wells and multilateral completions

SGR logging in horizontal wells and multilateral completions can offer several advantages for formation evaluation. First, SGR logging can help identify the lateral variations in lithology, mineralogy, and organic matter content along the wellbore, which can affect the reservoir quality and productivity. Second, SGR logging can help correlate the horizontal well with the vertical well or seismic data, which can improve the geological model and reservoir characterization. Third, SGR logging can help optimize the well placement and completion design by identifying the sweet spots and avoiding the non-productive zones.

3 Limitations of SGR logging in horizontal wells and multilateral completions

Despite its benefits, SGR logging in horizontal wells and multilateral completions also has some limitations and challenges that need to be considered. One of the main limitations is the borehole effect, which refers to the attenuation and scattering of gamma rays by the drilling fluid, casing, cement, and formation around the wellbore. The borehole effect can reduce the signal-to-noise ratio and introduce errors and uncertainties in the SGR measurements. Another limitation is the tool geometry and orientation, which can affect the depth of investigation and the resolution of the SGR measurements. The tool geometry and orientation depend on the wellbore trajectory, the tool conveyance method, and the tool centralization.

4 Workflow for SGR logging in horizontal wells and multilateral completions

To overcome the limitations and ensure the quality and accuracy of SGR logging in horizontal wells and multilateral completions, a systematic workflow is recommended. The workflow consists of four main steps: planning, acquisition, processing, and interpretation. In the planning step, the objectives, constraints, and risks of SGR logging are defined, and the optimal tool selection, conveyance method, logging speed, and depth control are determined. In the acquisition step, the SGR measurements are performed according to the plan, and the data quality is monitored and verified in real time. In the processing step, the SGR data are corrected for borehole effects, tool geometry and orientation, environmental factors, and calibration factors. In the interpretation step, the SGR data are analyzed and integrated with other data sources to derive the lithology, mineralogy, and organic matter content of the formation.

5 Quality control for SGR logging in horizontal wells and multilateral completions

Quality control (QC) is an essential part of SGR logging in horizontal wells and multilateral completions, as it can help detect and correct any errors, anomalies, or inconsistencies in the SGR data. QC can be performed at different stages of the workflow, such as before, during, and after the acquisition, processing, and interpretation. Some of the QC methods and tools include: checking the tool calibration and performance; comparing the SGR data with other logs, such as gamma ray, resistivity, or neutron; evaluating the borehole effects and corrections; validating the processing parameters and algorithms; cross-checking the interpretation results with core or cuttings data; and performing sensitivity analysis and uncertainty assessment.

6 Applications of SGR logging in horizontal wells and multilateral completions

SGR logging in horizontal wells and multilateral completions has various applications for formation evaluation, depending on the type and objective of the project. Some of the common applications include: mapping the lateral heterogeneity and variability of the formation; identifying the lithofacies and depositional environments; estimating the total organic carbon (TOC) and kerogen type; determining the clay content and types; evaluating the mineral composition and matrix properties; assessing the fracture density and orientation; optimizing the well placement and completion strategy; and monitoring the production performance and reservoir changes.

7 Here’s what else to consider

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